This invention concerns the production of ethers by reacting at least one alcohol with at least one mono-olefin comprising a double bond on a tertiary carbon atom.
It is well known to conduct this reaction in the presence of acid catalysts, and particularly in the presence of solid ion exchange resins in an acid form, the best results being obtained when using macroreticular solid sulfonic resins, e.g. those described in U.S. Pat. No. 3,037,052.
The alcohol is, for example, methanol or ethanol and the mono-olefin is a mono-olefin having a double bond on a tertiary carbon atom, for example isobutene, 2-methyl-1-butene, 2-methyl-2-butene, 2-methyl-1-pentene or 2-methyl-2-pentene. Olefin mixtures may be used; the olefins of the above-mentioned type, for example isobutene, are much more reactive than the bi-secondary olefins such, for example, as 2-butene or the primary-secondary olefins, for example 1-butene, so that it is possible to proceed with mixtures of olefins: the olefins having a tertiary carbon atom are almost the only ones which react, thus providing means for removing said olefins from a hydrocarbon stream, for example a C.sub.4 cut produced by steam-cracking or catalytic cracking and which may contain butadiene and/or saturated hydrocarbons.
The reaction of addition of alcohols onto the olefins which leads to the formation of ethers is a balanced and exothermic reaction.
It is therefore necessary on the one hand, to efficiently remove the reaction heat since the sulfonic resins do not withstand for a long time temperatures higher than 120.degree. C. and abrupt heat shocks are detrimental to the mechanical strength of the resin. On the other hand, it is obvious that, in order to achieve high conversion rates, it is preferable to conduct the reaction at low temperature but the efficiency is then limited by the resin activity itself.
Different proposals for conducting this reaction of addition of alcohols onto olefins have been made. It is known, for example, to pass the reactants in the liquid state through a fixed bed of catalyst particles. It has been found that, with regard to the mechanical strength of the resin and in order to avoid too high and irreversible increases of the pressure drop due to the packing of the resin, it is desirable to arrange the catalyst in a certain number of catalyst layers of small height and to cool down the liquid when passing from a catalyst lower to the next one. In another embodiment of fixed bed, the liquid is passed through several parallel tubes containing the catalyst and is cooled externally. However, in this case, the reactor is of a very complex and expensive type and, furthermore, it is difficult to avoid an uneven distribution of the liquid stream in the tubes, which results in a bad operation of the reactor and a quick deterioration of the resin.
The use of a reactor containing a catalyst dispersed in the reactants liquid phase does not provide for high olefin conversion rates unless reactors of excessive volume are used.
It has also been proposed, in order to obtain high conversion rates, to conduct the reaction in two serially arranged reactors with an intermediate separation of the product and to make use of a molar ratio alcohol/olefin higher than 1. In these various cases, however, the energy consumption for the distillation, either of the hydrocarbon cut, for example a C.sub.4 cut, or of the methanol or other alcohol in excess which must be recycled, is very substantially increased.
It has also been proposed to proceed to the reaction with two successive catalyst beds (German Federal Republic patent application No. 1934422). In the first bed, the catalyst is maintained in a dispersed state in the liquid by vaporizing one or more of the liquid constituents in order to remove partly the heat produced by the reaction. The second bed consists of the catalyst accumulated at the bottom of the reactor. The temperature conditions are accordingly substantially the same for the first and the second catalyst beds. The liquid circulates downwardly.
It has been found that this method suffers from a major disadvantage: the vapor phase is formed inside or at the contact of the resin particles and forms a layer surrounding them, impeding the free access of the reactants, thereby resulting in relatively poor conversions and selectivities and in a reduced life time of the catalyst. Another disadvantage results from the fact that the compound having the lower boiling point is vaporized and it is, in most cases, the diolefin which thus does not participate in the reaction or at least creates unbalanced conditions as far as the proportions of the reactants are concerned.
The French patent application No. 7831768 proposes to avoid these drawbacks by the following procedure:
A fresh liquid mixture of reactants comprising the alcohol and the olefin, is passed with a liquid recycle stream amounting to from 0.1 to 15 times the liquid rate of the fresh reactants, upwardly through a reaction zone (A) containing particles of solid catalyst of the sulfonated ion exchange resin type, in acid form, at a temperature of 60.degree.-120.degree. C. and selected lower than the boiling temperature of the most volatile constituent of the mixture, under the selected pressure; the feeding rate of this mixture is maintained at a level sufficient for expanding the volume of said bed by at least 2% and to disperse the particles, but insufficient for carrying away the catalyst to a noticeable extent out from said zone (A), the contact time being so selected as to convert 40-95% of said olefin; a first proportion of the liquid mixture resulting from the reaction is withdrawn, cooled down and fed back to the reaction zone (A) as recycle stream and another portion is fed to a second reaction zone (B), containing a solid catalyst in fixed bed of the same type as in zone (A), at a temperature of 30.degree.-70.degree. C., itself selected lower than the boiling temperature of the most volatile constituent, under the selected pressure.
The preferred temperature (zone A) is 75.degree.-100.degree. C. for the C.sub.4 hydrocarbons and 65.degree.-90.degree. C. for the C.sub.5 hydrocarbons.
The charge in the reaction zone (B) has accordingly the same composition as the effluent from reaction zone (A).
This process may be conducted in adiabatic reactors of a simple design and low cost. The heat produced by a reaction in the first reaction zone may be used partly to heat the reactants charge, the heat excess being optionally removed outside of the reaction zone by passing the effluent through a conventional heat exchanger before its recycling to the inlet of said zone. The stirring of the catalyst in the first reactor avoids the disadvantages relative to the increase of the pressure drop and, as a result of a better homogenization of the temperature suppresses the thermal shocks in the resin. The recirculation of the effluent, which is cooled down in an external exchanger, provides for a better control of the temperature and concentration gradients in the reaction zone and makes possible to operate with the resin at a higher temperature. It is observed that all of these particular operating conditions provide, in spite of the high temperatures in the first reaction zone, higher selectivities in MTBE and a longer life time of the resin.
The process gives satisfactory results in most cases.
It has however been observed that, when the hydrocarbon charge contains certain impurities, the life time of the catalyst is substantially reduced. Among the noxious impurities, there can be mentioned sulfur compounds and particularly the basic organic nitrogen compounds and certain metal ions, particularly those of alkali or alkaline-earth metals or those of iron, copper or lead.
These impurities may be originally present in certain charges or may be produced during preliminary treatments to which the charge has been subjected. For example, it is usual to treat the cracking fractions with inorganic bases, for example sodium hydroxide, or organic bases, for example an alkanolamine. The impurities may also be present in the alcohol, particularly in methanol.
As a general rule, all the compounds capable of reacting with the free sulfonic acid group of the resin may be considered as impurities. The problem arises particularly when the content in these impurities is higher than two parts per million by weight (expressed as NaOH).
It has thus been observed that the decrease in the catalyst activity in the first step makes it necessary to discontinue the operation as soon as this activity falls below a determined value, for example below 50% of the initial activity. Since the partially deactivated catalyst cannot be regenerated economically, it is necessary to discharge it entirely.
The process of the present invention avoids this disadvantage.